Digital switch-resistant multi-circuit line card providing direct connection to remote terminal

ABSTRACT

A multi-circuit emulating line card is installable in a single line card slot of the backplane of digital switch, and is configured to emulate the functionality of each of a plurality of digital switch line cards, respectively associated with plural digital subscriber circuits served by the switch. In the course of emulating these plural line cards, the multi-circuit line card provides connectivity between each digital subscriber circuit and a digital carrier communication link to plural digital subscriber loop circuits, such as BRITE cards of a remote terminal site. The line card of the invention also includes network and subscriber circuit-associated metallic link impedance simulation circuits for terminating a metallic test bus.

FIELD OF THE INVENTION

The present invention relates in general to communication systems, inparticular digital loop carrier (DLC) networks, and is directed to a newand improved line card installable in a single card slot of thebackplane of a digital switch serving a central office. Themulti-circuit line card of the invention is configured to emulate aplurality of digital switch line cards, respectively associated with aplurality of digital subscriber circuits served by the switch, and isoperative to provide connectivity between such digital subscribercircuits and a digital carrier communication link to plural digitalsubscriber loop circuits of a remote terminal site.

BACKGROUND OF THE INVENTION

In order to extend digital service to customer premises equipment (CPE)served by a remote terminal (RT), telephone networks customarily employa digital loop carrier (DLC) system, such as Ti carrier system for timedivision multiplexed data format (e.g., Bellcore standard TR-NWT-000397)diagrammatically illustrated in FIG. 1. In this ‘universal’ type of DLCsystem, opposite ends of the digital carrier communication link (the T1link) 10 are coupled to a central office terminal (COT) 12 and a remoteterminal (RT) 14, respectively. For multiplexing and demultiplexingdigital traffic conveyed over the T1 carrier link 10 with digitalsubscriber loops (DSL)s 21 of an associated switch 20 (such as an AT&T5ESS switch), the central office terminal 12 may contain a plurality ofdigital loop carrier circuits 13, such as basic rate interfacetransmission extender cards, commonly referred to as BRITE cards orU-BRITE cards. Each BRITE card 13 is coupled over a respective DSL 21 toone of a plurality of line cards 23 of the switch 20. In a complementarymanner, the remote terminal 14 may contain a plurality of BRITE cards15, respectively associated with DSLs 16 serving respective CPEs.

For digital switch applications, the digital subscriber loops 21 betweenthe switch 20 and the COT 12 become superfluous and may therefore beeliminated by terminating the carrier link to the remote terminal withthe switch. For this purpose, two Bellcore ‘integrated’ digital loopcarrier systems—TR-NTW-0008 and GR-CORE-303—have been proposed. Thepurpose of these systems is to reduce the cost per subscriber (byeliminating the subscriber loops between the switch and the COT), and toincrease bandwidth efficiency by concentration at the carrierfacilities.

However, each has its shortcomings. First of all, the TR-NTW-0008 systemdoes not provide for deployment of ISDN signalling. Secondly, theGR-CORE-303 system, which does deploy ISDN, involves a substantialmodification of the configuration of the switch, requiring a substantialcapital investment, that is passed on to the subscriber in the form ofhigher costs (which become prohibitive in areas of low subscriberdensity).

SUMMARY OF THE INVENTION

In accordance with the present invention, this problem is solved by anew and improved line card that is installable in a digital switchserving a central office, and is operative to emulate or ‘mimic’ thefunctionality of each of a plurality of digital switch line cardcircuits, that are respectively associated with a plurality of digitalsubscriber circuits served by the switch. In the course of emulatingthese plural line cards, the multi-circuit line card configuration ofthe invention provides connectivity between each digital subscribercircuit and a digital carrier communication link to plural digitalsubscriber loop circuits of a remote terminal site. Because it isinstallable in an individual one of the backplane card slots of theswitch, the multi-circuit line card of the invention provides asubstantial decrease in hardware, and therefore reduces the costassociated with providing ISDN service to low subscriber density areas.

As will be described, the multi-circuit line card according to thepresent invention comprises a card processor-controlled digital switchbackplane interface, that is made up of a plurality of backplaneinterface circuits—one for each (replaced) line card to be emulated. Asthe processor steps through or scans the backplane interface circuits,each circuit is operative to interface digital subscriber loop (DSL)signals (such as ISDN signals) and control signals with the respectivesignalling and control bus portions of the switch backplane.

For outgoing DSL signals from associated subscriber loops, a respectivebackplane interface circuit extracts the DSL data signals from thebackplane and supplies a response to the switch over the backplanecontrol bus. In the incoming direction, DSL signals for associatedsubscriber loops from the T1 carrier link are controllably placed ontothe backplane data bus.

To link DSL signals to and from the (T1) digital carrier link, eachbackplane interface circuit is coupled to a respective data formatcircuit of a data format interface. Each data format circuit formats DSLdata in accordance with a prescribed time division multiplexed dataformat (such as the above-mentioned Bellcore standard TR-NWT-000397) forapplication to the digital carrier link by a digital carrier interface(multiplexer—demultiplexer). At the remote terminal, the respective ISDNsubscriber channels transported by the carrier data stream are convertedinto plural ISDN subscriber loops for delivery to their associated CPEcircuits. In the incoming direction from the T1 link, each data formatcircuit extracts the DSL signals from the (TR 397) formatted data fromthe digital carrier interface and couples the DSL signals to anassociated backplane interface circuit.

In order to create the appearance of the presence of a network orcustomer interface to tip and ring leads of a metallic test bus, themulti-circuit line card of the present invention further includesselectively connectable network and subscriber circuit-associatedmetallic link impedance simulation circuits. Each impedance simulationcircuit may comprise a resistor-capacitor network, that is controllablyconfigured by the card's control processor to provide a prescribed testbus termination impedance. The metallic link simulation circuits serveto replicate loop impedance signatures that would normally be presentedby each replaced or phantom subscriber loop circuit between the switchand the central office terminal. This feature enables the multi-circuitline card of the invention to appear to the switch as a plurality ofsubscriber loops that had been extended through the conventional use ofa universal digital loop carrier system containing central officeterminal and remote terminal hardware.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 diagrammatically illustrates a conventional digital loop carriersystem;

FIG. 2 diagrammatically illustrates a digital loop carrier systemcontaining a digital switch contains the multi-circuit line cardaccording to the invention; and

FIG. 3 diagrammatically illustrates the configuration of a multi-circuitline card according to the present invention.

DETAILED DESCRIPTION

Before describing in detail the multi-circuit line card of the presentinvention, it should be observed that the invention resides primarily inwhat is effectively a prescribed arrangement of conventionalcommunication circuits and associated digital signal processingcomponents and an attendant supervisory control program therefor, thatcontrols the operations of such circuits and components. Consequently,the configuration of such circuits and components and the manner inwhich they are interfaced with other communication system equipmenthave, for the most part, been illustrated in the drawings by readilyunderstandable block diagrams, which show only those specific detailsthat are pertinent to the present invention, so as not to obscure thedisclosure with details which will be readily apparent to those skilledin the art having the benefit of the description herein.

Thus, the block diagram illustrations to be described are primarilyintended to show the major components of the system in a convenientfunctional grouping and processing sequence, whereby the presentinvention may be more readily understood.

FIG. 2 diagrammatically illustrates a digital loop carrier system of thetype shown in FIG. 1, in which a plurality of the lines cards 23 of thedigital switch 20 have been replaced by a single multi-circuit line card30 according to the present invention, a detailed diagram of which isshown in FIG. 3, to be described. Multi-circuit line card 30 ispreferably implemented as a gate array-configured application specificintegrated circuit (ASIC) and is installable in a single backplane cardslot of the switch 20. Respective (virtual) line card interfacefunctions that are executed by a set or plurality of interface andformatting circuits are operative to provide direct digital signallingconnectivity between a plurality of digital subscriber circuits servedby the switch 20 and the (T1) digital carrier communication link 10, afar or geographically remote end of which is terminated at the remoteterminal 14. As noted earlier, because it is installable in anindividual one of the backplane card slots of the digital switch 20,multi-circuit line card 30 provides a substantial reduction in switchhardware, and therefore a decrease in cost associated with providingISDN service to low subscriber density areas.

For this purpose, as diagrammatically illustrated in FIG. 3, in order toappear to the switch backplane as a plurality of installed line cardcircuits, by (virtually) replicating the digital signalling interfacefunctionality of a plurality of line card circuits that it replaces, themulti-circuit line card of the invention comprises a digital switchbackplane interface 40, comprised of a plurality of backplane interfacecircuits 41-1, 41-2, . . . , 41-N (where N=8 as a non-limiting example).Each backplane interface circuit 41 is preferably a gate arrayreplication of the DSL signal coupling functionality of a standard(replaced) line card, that interfaces digital subscriber loop (DSL)signals (such as but not limited to ISDN signals) and control signalswith the respective signalling and control bus portions of the switchbackplane, via backplane bus 35. Being virtual backplane interfacecircuits, rather than line cards physically installed in respective cardslots of the switch backplane, allows each backplane interface circuit41 to be individually programmable relative to any DSL time slot/cardslot assignment.

Backplane interface 40 and other components of the multi-circuit linecard are controlled by a card control processor (microcontroller) 100,with associated control signals being linked to the various circuitcomponents of the card by way of a control bus 101. The processor 100 isprogrammed to step through or scan the backplane interface circuits, inresponse to which digital subscriber loop (DSL) signals (such as ISDNsignals) and control signals are interfaced with the respectivesignalling and control bus portions of the switch backplane.

For outgoing DSL signals from associated subscriber loops, eachbackplane interface circuit 41-i is operative, under processor control,to extract their DSL data signals from the backplane and to generate aresponse to the switch over the backplane control bus. Conversely, asincoming DSL signals for delivery to associated subscriber loops arereceived from the T1 carrier link, and controllably placed onto thebackplane data bus, the associated interface circuit 41-i signals theswitch over the backplane control bus.

To perform data interface functionality for both replaced line cards ofthe switch and BRITE cards of the central office terminal, eachbackplane interface circuit 41 is configured to execute, under processorcontrol, embedded operations channel (EOC) processing that would beperformed by the BRITE cards, so that the multi-circuit line card mayprocess messages having an address of ‘1’ or a broadcast address.

For linking DSL signals to and from the (T1) digital carrier link 10,each backplane interface circuit 41-i is coupled over a DSL data bus 43to a respective data format circuit 51-i of a data format interface 50.Like the respective backplane interface circuits 41, each data formatcircuit 51 is preferably configured as a gate array replication of dataformat conversion functionality of a standard (replaced) BRITE card,that formats DSL data supplied thereto from the DSL data bus 43, inaccordance with a prescribed time division multiplexed data format (suchas Bellcore standard TR-NWT-000397 data format, as a non-limitingexample that provides for the transport of ISDN data), for applicationto the digital carrier link 10, by way of a digital carrier interface60. (At the remote terminal 14, the respective ISDN subscriber channelstransported by the TR 397formatted carrier data stream are convertedinto plural ISDN subscriber loops for delivery to their associated CPEcircuits).

The digital carrier communication link interface 60 comprises a digitalsignal packet multiplexer—demultiplexer, that is operative, under thecontrol of processor 100, to multiplex and demultiplex the (TR 397)formatted digital data communication signals with respect to the digitalcarrier communication link 10.

In the incoming direction from the T1 link 10, each data format circuit51 is operative to extract the DSL signals from the (TR 397) formatteddata as received over formatted data bus 61 from the digital carrierinterface and to couple the DSL signals over the data bus 43 to anassociated backplane interface circuit 41.

The multi-circuit line card further includes respective network andsubscriber circuit-associated metallic link impedance simulationcircuits 71 and 75. The purpose of these circuits is to ‘spoof’ theswitch, by creating the appearance of the presence of a network orcustomer interface to tip and ring leads of a metallic test bus 80, andthereby satisfy the switch's need to see reasonable loop parameters thatlook like the signature of a network or customer termination.

For this purpose, each metallic link impedance simulation circuit maycomprise a resistor-capacitor network, that is controllably configuredby the card's control processor 100 to provide a prescribed test bustermination impedance. The tip and ring leads of the metallic test bus80 are selectively coupled to the impedance simulation circuits 71 and75 by means of cascaded relay circuits 91 and 92 installed therebetween.Each relay circuit is selectively switched by control processor 100 toprovide a switchable circuit path between the tip and ring leads of thetest bus 80 and each of the selectively controlled impedance simulationcircuits 71 and 75.

Thus, the metallic link simulation circuits 71 and 75 serve to replicatethe corresponding impedance signatures that would normally be presentedby each replaced (‘phantom’) subscriber loop circuit between the switchand the central office terminal. This feature enables the multi-circuitline card of the invention to satisfy the switch, by appearing as aplurality of digital (e.g., ISDN) subscriber loops, that had beenextended through the conventional use of a universal digital loopcarrier system containing both central office terminal and remoteterminal hardware, as described previously.

As will be appreciated from the foregoing description, the abovediscussed desire to eliminate what are effectively superfluous digitalsubscriber loops between a digital switch and a central office terminal,by terminating the digital carrier link to the remote terminal directlywith the switch, yet without imposing a substantial cost penalty on thesubscriber, is successfully addressed by the multi-circuit line card ofthe invention, which is installable in a single card slot of digitalswitch and is operative to effectively mimic the functionality of eachof a plurality of digital switch line card circuits it replaces. Inemulating the replaced line cards, the multi-circuit line cardconfiguration of the invention provides connectivity between eachdigital subscriber circuit and a digital carrier communication link toplural digital subscriber loop circuits of a remote terminal site.Because it is installable in an individual one of the backplane cardslots of the switch, the multi-circuit line card of the inventionprovides a substantial decrease in hardware, and therefore reduces thecost associated with providing ISDN service to low subscriber densityareas.

While we have shown and described an embodiment in accordance with thepresent invention, it is to be understood that the same is not limitedthereto but is susceptible to numerous changes and modifications as areknown to a person skilled in the art, and we therefore do not wish to belimited to the details shown and described herein, but intend to coverall such changes and modifications as are obvious to one of ordinaryskill in the art.

What is claimed is:
 1. In a digital switch associated with a firstterminal site of a communication network, said digital switch beingoperative to provide connectivity between a plurality of digitalsubscriber circuits coupled thereto and a digital carrier communicationlink connected to a remote terminal site, said remote terminal sitehaving a plurality of digital subscriber connection circuits servingrespective digital subscriber loops connected thereto, the improvementcomprising a multi-circuit line card installable in said digital switchand being configured to emulate a plurality of digital switch line cardcircuits, respectively associated with said plurality of digitalsubscriber circuits, and providing connectivity, over said digitalcarrier communication link, between each of said digital subscribercircuits and said plurality of circuits of said remote terminal site,and thereby providing digital communication service for said respectivedigital subscriber loops.
 2. The improvement according to claim 1,wherein said multi-circuit line card comprises: a digital switchbackplane interface having a plurality of backplane interface circuitscoupled to interface digital subscriber loop data signals with saidplurality of digital subscriber circuits, and a digital carriercommunication link interface coupled with said digital switch backplaneinterface and said digital carrier communication link, and having aplurality of digital communication interface circuits coupled tointerface said digital subscriber loop data signals between said digitalswitch backplane interface and said digital carrier communication link.3. The improvement according to claim 1, wherein said digital subscriberloop signals comprise integrated service digital network communicationsignals.
 4. The improvement according to claim 1, further includingmetallic link interface circuits for selectively terminating a metallictest bus.
 5. The improvement according to claim 1, wherein said digitalcarrier communication link interface is configured to multiplex anddemultiplex digital data communication signals with respect to saiddigital carrier communication link.
 6. The improvement according toclaim 5, wherein said digital data communication signals compriseBellcore standard ISDN signals.
 7. The improvement according to claim 5,wherein said digital data communication signals comprise TR-NWT-000397formatted ISDN signals.
 8. A multi-circuit line card adapted to beinstalled in a line card slot of a backplane of a digital switch, andbeing operative to replace a plurality of digital switch line cards thatwould otherwise be installed in respective line card slots of saidbackplane, said digital switch being associated with a first terminalsite of a communication network and being operative to provideconnectivity between a plurality of digital subscriber circuits coupledthereto and a digital carrier communication link connected to a remoteterminal site, said multi-circuit line card comprising: a backplaneinterface configured to emulate said plurality of digital switch linecard circuits, respectively associated with said plurality of digitalsubscriber circuits; and a digital carrier communication link interface,coupled to said a backplane interface, and being operative to effectdigital carrier communication connectivity over said digital carriercommunication link between each of said plurality of digital subscribercircuits coupled to said digital switch and said remote terminal site,so as to provide digital communication service between said respectivedigital subscriber loops served thereby and said plurality of digitalsubscriber circuits coupled to said digital switch.
 9. The multi-circuitline card according to claim 8, wherein said backplane interfacecontains a plurality of backplane interface circuits, each of which isassociated with a respective line card replaced by said multi-circuitline card, and is operative to interface digital subscriber loop (DSL)signals and control signals with respective signalling and control busportions of said backplane.
 10. The multi-circuit line card according toclaim 9, wherein said digital carrier communication link interfacecontains a plurality of digital carrier communication link interfacecircuits, each of which is associated with a respective one of saidbackplane interface circuits, and is operative to format said DSLsignals in accordance with a prescribed time division multiplexed dataformat for application to said digital carrier communication link. 11.The multi-circuit line card according to claim 10, wherein said digitalcarrier communication link interface includes amultiplexer—demultiplexer, coupled to said plurality of digital carriercommunication link interface circuits, and being operative to providetime division multiplexed digital communication connectivity offormatted DSL signals over said digital carrier communication link withsaid remote terminal.
 12. The multi-circuit line card according to claim8, further including metallic link interface circuits for selectivelyterminating a metallic test bus.
 13. The multi-circuit line cardaccording to claim 12, wherein said metallic link interface circuitscomprise selectively connectable network and subscribercircuit-associated metallic link impedance simulation circuits, whichare controllably connectable to a metallic test bus, so as to appear tosaid switch as a plurality of subscriber loops extended through adigital loop carrier system containing both central office terminal andremote terminal hardware.
 14. The multi-circuit line card according toclaim 9, wherein said digital subscriber loop signals compriseintegrated service digital network communication signals.
 15. Themulti-circuit line card according to claim 8, wherein said digitalsubscriber loop signals comprise Bellcore standard ISDN signals.
 16. Themulti-circuit line card according to claim 11, wherein said formattedDSL signals comprise TR-NWT-000397 formatted ISDN signals.
 17. Amulti-circuit line card adapted to be installed in a line card slot of abackplane of a digital switch, and being operative to replace aplurality of digital switch line cards that would otherwise be installedin respective line card slots of said backplane, said digital switchbeing associated with a first terminal site of a communication networkand being operative to provide digital communication connectivitybetween a plurality of digital subscriber circuits coupled thereto and adigital carrier communication link connected to a remote terminal site,said multi-circuit line card comprising: a digital backplane interfaceconfigured to emulate said plurality of digital switch line cards andappear to said digital switch as a plurality of digital switch linecards, respectively associated with said plurality of digital subscribercircuits served by said digital switch; and a digital carriercommunication link interface, coupled to said digital backplaneinterface, and being operative to extract digital subscriber loop (DSL)data signals provided by said digital backplane interface forapplication to said digital carrier communication link in accordancewith a prescribed time division multiplexed data format, so as to effectdigital carrier communication connectivity over said digital carriercommunication link between each of said plurality of digital subscribercircuits coupled to said digital switch and said remote terminal site,and thereby provide digital communication service between saidrespective digital subscriber loops served thereby and said plurality ofdigital subscriber circuits coupled to said digital switch.
 18. Themulti-circuit line card according to claim 17, wherein said digitalcarrier communication link interface is operative to extract digitalsubscriber loop (DSL) data signals provided by said digital backplaneinterface for application to said digital carrier communication link asTR-NWT-000397 formatted ISDN signals.
 19. The multi-circuit line cardaccording to claim 17, further including metallic link simulationcircuits that are operative to replicate impedance signatures that wouldnormally be presented by respective ones of replaced digital switch linecard emulated by said digital backplane interface.